(1) Field of the Invention
The present invention relates to a method and apparatus for the ultrasonic flaw detection of a T-welded portion of a steel product.
More particularly, the present invention relates to a method and apparatus for the on-line ultrasonic flaw detection of a welded portion of a shaped steel product such as an H-beam.
(2) Description of the Prior Art
A steel product having a T-welded portion, for example, an H-beam is prepared by resistance welding of two steel strips corresponding to flanges and a steel strip corresponding to a web. For control of the quality of this welded H-beam, the welded portion should be inspected as specified in ASTM A-769.
As means for inspecting and guaranteeing the quality of welded H-beams, there have heretofore been adopted the following off-line methods; (1) a method in which the end face of an H-beam is directly inspected with the eye, (2) a method in which a test specimen having a length of 30 mm is sampled from a welded H-beam and this test specimen is subjected to an I-type tensile test, and (3) a method in which a test specimen having a length of 30 mm is sampled from a welded H-beam, the upper flange and web are bent down at a predetermined angle with the lower welded portion being as the center while the lower flange is fixed, to thereby impose a load on the lower welded portion, and the lower welded portion is inspected with the eye.
In these methods, however, since the off-line inspection system is adopted, it sometimes happens that when a defect is detected at the inspection stage, considerable amounts of defective products have already been produced in the production line, resulting in reduction of the yield. Moreover, this inspection is a laborious work and the quality is not completely guaranteed because the test is a sampling test.
Under such background, we conducted research with a view to developing a method for inspecting the entire surface of the welded portion in the production line by utilizing the ultrasonic flaw detection method.
However, it was found that it is very difficult to precisely inspect a T-welded portion of a shape steel according to the conventional ultrasonic flaw detection technique.
FIG. 1 schematically illustrates a method of the flaw detection of a T-welded portion according to the partial water penetrating test technique.
In the conventional method illustrated in FIG. 1, flanges 1F and 1F' of an H-beam 1 are horizontally arranged, and parallel ultrasonic beams 2 are applied downward to the upper face of the upper flange 1F. The flaw detection system comprises a frame member 3 forming, together with the top face of the flange 1F, a vessel containing a medium 4 such as water therein and a detecting probe 5 immersed in the medium. According to this method, the parallel ultrasonic beam 2 are incident on the welded portion from the detecting probe 5, and a flaw echo is checked.
In this method, however, since the ultrasonic beams 2 are simultaneously incident on the non-welded portion as well as on the welded portion, a wave form in which the flaw echo is overlapped on the bottom face echo is produced, and discrimination of the flaw echo is very difficult and a complicated electronic processing system becomes necessary for detection of the flaw echo. This disadvantage will now be described in detail with reference to FIG. 2. FIG. 2 is a schematic view showing the stage where parallel beams from the detecting probe 5 are incident on the welded portion 6 including a flaw, and the lower portion thereof shows a cathode-ray tube wave form of reflected waves, which is illustrated according to the single side deflection method. In case of T-shape welding, since the flaw is present substantially on the same level as the bottom face of the flange 1F, as shown in FIG. 2, there is produced a wave form in which a flaw echo F is overlapped on a bottom face echo B, and the increase of the echo level by the flaw echo F is small and discrimination of the flaw echo F is difficult. In FIG. 2, S represents a surface echo.
Moreover, if the surface of the flaw portion is rough and is inclined to the incident direction of the beams, the echo height of the flaw echo is low, and reduction of the detection precision is often caused.